By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

Thread cutting problem

All Topics | Latest Posts

Search for:  in Thread Title in  
Steve Tyson28/11/2020 17:52:56
9 forum posts

For 3/16" x 40 tpi your material is undersize at 0.180", try 0.188".smiley

Stuart Bridger28/11/2020 17:57:52
502 forum posts
28 photos
Posted by old mart on 28/11/2020 17:46:08:

If the die is not split, it is only good for cleaning up an existing thread. If the die is split, the die holder has to have a big enough bore to allow the die to be expanded by the central screw, for the first pass. Then the first screw is backed off a bit and the other two screws done up lightly on the die for the second pass. The quality of dies varies from excellent to christmas cracker, you may have the latter and could be trying to accomplish the impossible.

With respect, I have to disagree, I have a quality set of metric dies that are not split and are toleranced (6g). They cut a an excellent thread.

DiogenesII28/11/2020 18:17:17
167 forum posts
70 photos

Might be worth checking that tightening the screw(s) isn't causing the die to cock in the holder, preventing it from sitting completely flat - I've come across dies with poorly formed or incorrectly positioned screw seats now & again..

Buffer28/11/2020 19:30:42
207 forum posts
84 photos

Thanks to everyone for the advice and replies.

Can anyone tell me where you find out or how you work out what size bar you should start with when cutting a thread? I thought you had to have the bar slightly under size as the die doesn't just cut but also forces the metal out a bit.

Andrew Johnston28/11/2020 19:53:02
5824 forum posts
662 photos

I use the nominal thread diameter. That's true whatever method is used to form the thread - single point screwcutting, Coventry diehead, or split dies by hand and under power on the repetition lathe.


Buffer28/11/2020 21:31:10
207 forum posts
84 photos


What is the nominal thread diameter?


Ian Johnson 128/11/2020 21:38:34
313 forum posts
88 photos

Funnily enough I actually made three stainless 10-32 UNF washer faced bolts tonight for my Norton air filter box, but didn't have a die, so I used a 2BA die instead which is near enough.

I always make the diameter a bit smaller when die cutting stainless threads, so ended up at 0.182 not 0.185 as it should be for 2BA, everything worked out great and fits great too. Although when machining brass its nice and easy to thread so I would make the diameter to size.

So don't worry too much about outside diameter if it's under by a few thou, if you get 75% thread contact area that is certainly good enough for model engineering.


Buffer28/11/2020 21:45:25
207 forum posts
84 photos

I'm now wondering if the threads are being stripped as I am trying to get the thread started. In other words maybe it is rotating without pulling itself along which is stripping the start of the thread clean.

Paul Lousick28/11/2020 22:05:23
1637 forum posts
610 photos

I had a similar problem cutting M2 threads in brass. The die stripped the material instead of cutting a thread when starting. Not sure but die could have been sharp enough and I did not have another.

Problem solved by making the part to be threaded longer and reducing the diameter at the end so the die could start easily (cutting a half height thread). This allowed me to continue the cut on the full diameter. The extra material at the end then removed.


Ian Johnson 128/11/2020 22:05:25
313 forum posts
88 photos

Try 10 thou undersize and see what happens


Andrew Johnston28/11/2020 22:59:19
5824 forum posts
662 photos
Posted by Buffer on 28/11/2020 21:31:10:

What is the nominal thread diameter?

By nominal I mean the thread size. So in the case of your 3/16" x 40 thread I'd turn to 3/16", without worrying about the 5 tenths, say 0.187".

I don't think having the blank at nominal thread diameter is the problem. I've recently made a batch of wick feed oilers for my traction engines:


The threads on the spigot underneath the body are 1/4" x 40. They were screwcut using a full form insert; the OD measures 0.25" give or take the odd few tenths, some over some under. Likewise the lids bottom left have a 7/16" x 32 screwcut thread. They measure 0.4375" on the OD.

The interesting part is the 5/32" x 40 threads on the copper tubes. The tube OD measures 0.1575". A 2.5mm drill slips neatly inside. That gives a wall thickness of 0.0295". A 40 tpi Whitworth thread has a depth of about 0.016" so that leaves 0.0135" minimum wall thickness after threading. When I made the first batch of these oilers I screwcut the 5/32" x 40 threads. But it was a pain as slightly too much depth of cut and the remaining thin wall gave way. So this time I resorted to a die in a tailstock dieholder. It worked fine and the OD of the resultant thread is 0.1575", so slightly over nominal. But they fit nicely in the mating tapped holes. The threads are properly formed for the full length and without breaking the remaining thin wall. That's why I don't think making the stock undersize is the solution. It's perfectly possible to cut nice threads starting at nominal diameter with a die. Reducing the diameter may appear to help, but it's covering up the problem not solving it. The die I used was part of a present (Triangle brand) some 50 odd years ago and the tailstock dieholder came from Arc, so neither are top notch professional (*), but they did the job fine. A few questions:

Where did the die and tailstock holder come from?

Have the die and/or holder been used satisfactorily in the past?

Was the die opened out to its maximum to check the fit before adjusting?

Have you tried the same setup on something more forgiving like brass?


(*) Apologies to Ketan - I'm more than pleased with the tailstock dieholder set from Arc and it does what I expect without fuss, with one minor annoyance.

Buffer29/11/2020 08:58:13
207 forum posts
84 photos


The tailstock holder is a homemade one but it works very well normally. The die I'm not sure about but most of my tackle comes from Tracy Tools so should be good. I'm sure I did have it at its maximum opening I wound the centre screw in first then just nipped the other two to stop it falling out. I will try it on some brass and see.


Howi29/11/2020 09:46:59
289 forum posts
19 photos
Posted by Buffer on 28/11/2020 21:45:25:

I'm now wondering if the threads are being stripped as I am trying to get the thread started. In other words maybe it is rotating without pulling itself along which is stripping the start of the thread clean.

hi buffer, have you tried reversing the die, NOT all cut(well) in both directions. With regard to nominal size, I have always used the same size i.e 3/16 die, 3/16 material, if machining down to a size then go a few thou under.I usually put a slight taper/bevel at the end of the material to give the die sometrhing to start on.


Hopper29/11/2020 09:56:50
5034 forum posts
111 photos
Posted by Buffer on 28/11/2020 21:45:25:

I'm now wondering if the threads are being stripped as I am trying to get the thread started. In other words maybe it is rotating without pulling itself along which is stripping the start of the thread clean.


Are you pushing on the unclamped tailstock with your right hand to force the die onto the job in the direction you want it to travel?

And are you turning the chuck by hand rather than under power?

With a back and forth motion to break the chips as you go?

With suitable tapping compound on the job?

If yes to all of the above, you may have a duff die. Test it on a bit of scrap held in the vice and the die in a regular dieholder by hand.


Edited By Hopper on 29/11/2020 09:57:36

Edited By Hopper on 29/11/2020 09:59:02

Nigel McBurney 129/11/2020 10:21:15
775 forum posts
3 photos

The problem could be poor manufacture of the die,I have had a tap from a well known supplier which jammed solid in the hole,it had no clearance ,or possibly negative clearance,replaced without question,and the replacement worked perfectly,i also have a set of set of ME taps and dies,one of the dies just will not cut,others in the set are ok,this was one of those exhibition offers from many years ago again poor manufacture no doubt from the far east,again a well known uk vendor. Again I bought a set of drills at an exhibition, a previous similar .set from the same vendor had been very good. this second larger set was very poor,some drills ground off centre,others with negative clearance. I had lost the receipt and could not be bothered to send them back ,so used them for making counterbores and a lot were resharpened and ground backed off for brass, next year I tackled the chap on the exhibitors stall,he admitted that the had received a poor batch and asked if i had brought them along,and he would replaced them,could not fault that,I just told him that I had used them for other purposes, I did buy a same size set from another source of a known make ,only snag they were 5 times the cost.

Andrew Johnston29/11/2020 10:52:42
5824 forum posts
662 photos

PM sent - Andrew

IanT29/11/2020 12:53:49
1744 forum posts
164 photos

Coming a bit late to this thread - but I used to have this problem too, although maybe not quite so extreme.

I do most of my small brass fittings on my EW lathe and leave it set up normally with a 40tpi screw-cut in mind (but not normally engaged). I use a handle to turn the lathe manually and cut a fairly deep (but not full) thread. If possible, I also allow some extra length on the part being threaded. I have a very thin parting tool that I use to make a slight groove at end of the cut when the part needs to screw up flush.

I then run a die over the part, finishing the thread form. If you have a split die, open it up a little before the first run down the thread - then check that a "nut" tapped with the tap you intend to use for any mating part - will also run down the thread. If not, allow the die to close up a little and run it down again.

The part screw-cut will help the die run on true (I do use tailstock die-holders but on fine threads they still need help). If you have made the part 2-3 threads over length, you can then face the extra length off and slightly chamfer the end. If you leave your "test" nut on, it will clean up your thread-end as you remove it.

Hope this helps.



Edited By IanT on 29/11/2020 12:57:01

Rod Renshaw29/11/2020 12:56:32
230 forum posts
2 photos

Lots of good points made by posters above.

If the diameter and the material of the stock are OK, and the technique and lubricant are OK, then the only thing left is the die which is either faulty or it's being presented to the stock off centre or at an angle?

I understand split dies are intended for use by hand held methods. Good ones will generally be concentric, between cutting edges and outside diameter, but cheap ones may not. I don't think there is any cure for a die which has the central hole and teeth at an angle to the outside faces.

Tailstock dieholders are great but they need to have some slack in them somewhere to allow the die to line itself up with the stock, some are just too rigid, "too well made!." and as has been said, a small die cannot be expected to pull a heavy tailstock.


David Colwill29/11/2020 13:16:07
703 forum posts
38 photos

Years ago I made up a guided die stock. This is basically a die stock that can take different bushes to suit different threads. In your case the bush could be sized to the larger diameter of the stock. I always find this has a much better feel than when aligned by other methods.

Harold Hall's version



Edited By David Colwill on 29/11/2020 13:16:39

Andrew Johnston29/11/2020 14:17:15
5824 forum posts
662 photos
Posted by Rod Renshaw on 29/11/2020 12:56:32:
I understand split dies are intended for use by hand held methods.

Indeed, but split dies also work fine under power. This is a sample holder and reversing dog clutch that I use on my repetition lathe:


I've just used it to cut some 2-56 UNC threads in brass at 500rpm, as I didn't have suitable chasers for the Coventry diehead.


All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Support Our Partners
Eccentric July 5 2018
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest